In modern motor vehicles, the energy storages for supplying safety-critical electrical consumers, e.g., break-by-wire, steer-by-wire, start-stop systems, or electrohydraulic brake systems, must be constantly monitored and their performance must be tested. An electronic battery management system monitors the performance of the batteries, thereby decisively supporting the demand-oriented control of the vehicle systems. A variable, significant for the performance capacity of a battery, is its state of charge (SOC). Based on the SOC value and the SOH value (state of health), a decision is made as to whether pure comfort functions such as air conditioner, radio, or power windows are to be switched off or whether other measures have to be taken in order to maintain safety-relevant functions such as ABS, ESP, or the brake booster.
Different methods for forecasting the performance of batteries are known from the related art. Variables, significant for the performance capacity of the batteries, such as the state of charge and the battery-internal resistance, for example, are determined by analyzing the open-circuit voltage, as well as via voltage and current measurements during start, or via model-based state observation under continuous measurement of voltage, current, and temperature, and a power output of the battery to be expected is predicted using these variables. Such a model-based method is known from German Patent Application No. DE 101 06 508. Among other things, the state of charge SOC is continuously estimated during vehicle operation or determined on the basis of an estimated open-circuit voltage of the battery.
During rest periods, in which the battery is essentially off-load, the open-circuit voltage is measured and used to draw conclusions about the state of charge (SOC).
Problematic is the occurring acid stratification in the battery. During charging, of a lead-sulfuric acid battery for example, the sulfuric acid emerging on the plates is heavier than the surrounding diluted sulfuric acid so that it sinks into the lower part of the battery due to gravitation. Therefore, during rest periods without electrolyte movement, the acid density is reduced in the upper part of the battery and increased in the lower part of the battery. This results in a sulfuric acid density gradient, which is known as acid stratification. The acid stratification should not be maintained permanently, because it results otherwise in irreversible damage to the battery. Another disadvantage of acid stratification is the fact that open-circuit voltages are measured to be too high in the rest period so that states of charge (SOC values) are determined to be too high. Consequently, the instantaneous state of the battery is evaluated too optimistically so that the vehicle containing the battery is possibly no longer able to be started, even though the state of charge has been previously determined to be sufficiently high.